A magnetic disk storage apparatus (which may be simply referred to as a “disk drive”) whose representative example is a hard disk drive (HDD) comprises a servo system for positioning a head at a target position on a disk. The servo system demodulates (calculates) a position signal (position information) indicating a position of the head on the disk based on a servo pattern (servo data) recorded in a servo area (servo wedge) on the disk.
The position signal includes a track address (Gray codes) for identifying a servo track number on the disk and a position error signal (PES) indicating a relative position of the head with respect to the servo track. Servo demodulation is hereinafter intended as demodulation of the position error signal (PES).
As the servo pattern, there are mainly a null servo pattern and an area servo pattern. The null servo pattern is comprised of two burst patterns, i.e. burst signal patterns having different magnetization patterns. Accordingly, a disk drive which uses the null servo pattern can reduce a servo area as compared to the case of using the area servo pattern, and thus, a disk drive with the null servo pattern has the advantage of being able to secure more data areas.
The servo system demodulates the null servo pattern from a read signal read by a reader (or read head). Further, the servo system is configured to correct the demodulated signal and to execute servo signal demodulation for demodulating the position error signal (PES) indicating the position of the head (reader). In the servo signal demodulation, in order to demodulate the position error signal which correctly indicates the position of the reader, linearity compensation, which corrects an error of the demodulated position with respect to the actual position, is especially important. In the linearity compensation, it is important to obtain an optimal compensation coefficient (or optimal compensation parameter) of the correction function.
In the past, correction functions using compensation coefficients were proposed, and methods for determining optimal compensation coefficients for those correction functions were also proposed. However, optimal compensation coefficients depend on a magnetization pattern distribution on a disk and a head sensitivity distribution of a read head. The head sensitivity distribution varies depending on an azimuth, a flying height, or pole tip protrusion (PTP). Especially, it has been confirmed that the head sensitivity distribution varies depending on the PTP, and by this change, a servo demodulation error increases significantly. Therefore, after shipment of a disk drive, it is preferred that a servo system monitors an optimal compensation coefficient and updates the optimal compensation coefficient in a relatively short time depending on the change in the PTP, etc. By such an update, realizing servo signal demodulation for demodulating a correct position error signal is enabled.